This proposal is to request support for a Keystone Symposia meeting entitled "TH17 Cells in Health and Disease", organized by Chen Dong, Vijay Kuchroo and Brigitta Stockinger, which will be held in Vancouver, British Columbia, Canada from February 5 - 10, 2009. T lymphocytes are important mediators of autoimmune diseases and their tolerance to self-antigens is tightly regulated. Regulatory T (Treg) cells maintain immune tolerance. Recently, a novel subset of CD4+ T cells that produce IL-17 (named Th17) has been identified and shown to be highly pathogenic in many autoimmune diseases. Surprisingly, there appears to be a reciprocal relationship between Th17 and Treg cells in their generation and function. Therefore, T cell biology needs to be further re-examined in the context of molecular pathways that are required for their generation, mechanisms that are involved in their antagonism, and function. The goal of this symposium is to bring the leaders in the field together to assess the increased complexity in the generation of pathogenic and regulatory T cells at the cellular, molecular and organismal levels and discuss the potential impact of these new findings on the development of treatments of human autoimmune diseases. The immune system is delicately regulated to effectively combat against various infectious agents while preventing autoimmune reactions against host tissues. CD4+ T lymphocytes are crucial organizers of immune responses. Recently, a novel subset of CD4+ T cells that produce IL-17 (named TH17) has been identified and shown to be highly pathogenic in many autoimmune diseases. This is the first major symposium devoted to TH17 cells, and likely will inspire further development of the field and translate the current findings into clinical treatments.